During sealed ampoule annealing ofn‐type (S,Se,Sn) InP crystals at 550 °C, an unexpected diffusion front was observed whose apparent diffusivity exceeded that of the rapidly diffusing Zn impurity by about two orders of magnitude. Using luminescence techniques, this diffusion front was correlated with changes in the ionized impurity level. In this letter, we show that for annealed InP:S (2×1018cm−3) crystals the donor profile obtained from cathodoluminescence (CL) intensity measurements corresponds to the34S profile obtained by secondary ion mass spectroscopy. Although dislocations are generally paths along which impurities can rapidly migrate, CL imaging shows that dislocations impede rather than enhance sulfur out diffusion. Further study is required to determine the rapid out‐diffusion mechanism.